This invention relates to a method of preparing a resistance thermometer having a resistance temperature detector element provided in a metallic sheath and operating by measuring the electric resistivity of the element, the resistance of which is a known function of its temperature.
An example of the conventional resistance thermometer of the above-mentioned type is schematically illustrated in FIG. 7, in which indicated generally by the reference numeral 1 is a resistance temperature detector element (hereinafter referred to as "RTD element"). The RTD element includes a mandrel 2 formed of an insulating material such as a glass, and two resistance wires 3, such as platinum wires, wound helically on the mandrel 2. The resistance wires 3 have respective terminal ends joined with each other so that the RTD element has a predetermined electric resistivity. The mandrel 2 and the winding 3 are surrounded by a cover 4 formed of a suitable insulating material such as glass. Designated by the reference numeral 5 are terminal leads of the winding 3. The terminal leads 5 extend from the cover 4 and are electrically connected to associated conductors 7 by means of intermediate connecting wires 6 which have a larger cross sectional area than that of the terminal leads 5 and which are in the form of thin ribbons. The RTD element 1 and the conductors 7 are located within a metallic sheath 8 of a very small diameter and supported and insulated from the metallic sheath 8 by means of a firmly compacted mass 9 of a finely divided, insulating material such as a metal oxide, for example, magnesium oxide. The conductors 7 have respective terminal ends connected to the circuitry (not shown).
The resistance thermometer of the above-mentioned type has been generally manufactured so far by the following method: The conductors 7 are inserted in a first metallic sheath 10 and supported in position by filling the space within the sheath 10 with the insulating material 9. The conductors 7 are joined to the terminal leads 5 of the RTD element using the intermediate connecting wires 6 by welding. A second metallic sheath 11 is then connected to the first sheath 10 by arc welding while maintaining both sheaths 10 and 11 in abutting engagement with each other. The reference numeral 13 in FIG. 7 designates a welded portion between the first and second sheaths 10 and 11. The space within the second sheath 11 is then filled with the insulating material 9 to fix the RTD element 1 in position. Finally, the open end of the second sheath 11 is closed by a closure 12 to complete the resistance thermometer.
The above-described conventional method has been found to have a drawback because the arc welding for integrally connecting the first and second sheaths 10 and 11 is apt to cause the melting of the conductors 7 and intermediate connecting wires 6 or junctures therebetween, which results in the electrical disconnection between the RTD element and the circuitry. In order to prevent such melting, therefore, it is necessary to use metallic sheaths having a large diameter and to provide a sufficient space between the conductors 7 and the sheaths. This is, however, disadvantageous because it is desirable that the metallic sheath have as small a diameter as possible.